Both are OK, but "J'en parle" is kinda undefined, so if the wording of your sentence makes the usage of "en" ambiguous, it's better to use "Je parle d'eux" or "Je parle d'elle"...

Example:

Mon petit frère tire les cheveux de ma petite soeur et je m'en moque.

Who are you mocking? The little brother? The little sister? The fact that he is pulling her hair? Following the grammatical rules, you should refer to the last "antecedant", that is the last possible solution in the sentence (=the sister). But that's not what you mean: really, you want to mock the little brother. So you remove the ambiguity by saying: